What do the beads mean on Annabeth’s necklace? Chapter 16

Annabeth's beads commemorate the most important events of the summer.

"Yeah," she said. "Every August, the counselors pick the most important event of the summer, and they paint it on that year's beads. I've got Thalia's pine tree, a Greek trireme on fire, a centaur in a prom dress—now that was a weird summer...."